1. Field of the Invention
The present invention relates to an ink-jet printing apparatus and a cleaning control method of the same and, more particularly, to an ink-jet printing apparatus having a function of cleaning the ink discharge surface of a printing head for printing an image on a printing medium by discharging an ink, and a cleaning control method of the apparatus.
2. Description of the Related Art
An ink-jet printing method has been conventionally used in printing units of printers and copying machines because of, e.g., its low noise, low running cost, ease of apparatus miniaturization, and ease of color printing.
In printing according to this ink-jet printing method, a printing head discharges ink droplets onto a printing medium such as an OHP film. Therefore, fine ink drops (mist) formed in addition to the discharged ink droplets or ink drops discharged onto the printing medium and splashing the discharge surface of the printing head. If a large amount of this splashing ink gathers on discharge openings of the printing head or if foreign matters such as paper dust particles adhere to this ink, the ink discharge is interfered with. Consequently, the ink is discharged in an unexpected direction or no ink droplets are discharged to cause discharge errors.
To solve this problem, therefore, conventional ink-jet printing apparatuses include a mechanism for wiping the discharge opening surface of a printing head with a blade formed by an elastic member such as rubber by using relative motion between them. This mechanism functions as a means for removing an unnecessary ink, paper dust, and the like sticking to the printing head by mist and/or splashes of ink drops from a printing medium.
This wiping operation is an important technique to improve the reliability of an ink-jet printing apparatus. Additionally, as a means for removing foreign matters sticking to the discharge opening surface of a printing head without being completely removed by the wiping operation, an ink-jet printing apparatus has a suction recovering mechanism constructed of a cap covering the discharge surface of the printing head and a suction pump connected to this cap. An ink is forcedly drawn out from an ink discharge nozzle of the printing head by the negative pressure generated by the suction pump. Since this removes an ink with an increased viscosity and/or foreign matters, normal discharge is recovered.
The wiping operation of the conventional ink-jet printing apparatuses is so controlled as to be executed in accordance with one or both of the printing time and the number of dots (printing dots) of an ink discharged from a printing head. This wiping operation is generally so set that the operation is executed before the wetness of an ink adhered to the discharge opening surface of a printing head brings about discharge errors.
Furthermore, in Japanese Patent Laid-Open No. 7-125228, the number of printing dots and the printing time are measured, and the timing of the wiping operation during printing is controlled on the basis of each of the number of printing dots and printing time measured.
This prior art, however, performs only control for determining whether the wiping operation is executed during image printing.
Accordingly, in a serial type ink-jet printing apparatus which prints images by reciprocating, in the direction (main scanning direction) perpendicular to the conveyance direction (sub-scanning direction) of a printing medium, a carriage mounting a printing head having an array of a plurality of ink discharge nozzles, if the wiping operation is executed during printing, the printing density of a printed portion immediately after this wiping operation changes. As a consequence, a density variation occurs in the printed image.
This density variation also occurs when each pixel on a printing medium is printed by overlapping a plurality of ink droplets or when printing is performed using a multipath printing method (also called a fine printing method) of forming an image by scanning the same region a plurality of number of times by a printing head. If, however, printing is executed in predetermined rhythm, the penetration of an ink into a printing medium also becomes uniform, and the image density stabilizes. In contrast, if the predetermined rhythm breaks owing to the wiping operation during printing or the like, the penetration of an ink into a printing medium also changes discontinuously to cause an ink density change. This density change is presumably visually sensed as a variation.